The mechanical attachment of workpiece components such as metal sheets and parts by the use of rivets has been known for some time. Conventionally, rivet holes are predrilled or stamped through two or more components and aligned. A rivet provided with a preformed or premanufactured rivet head and a cylindrical rivet body or shaft is then inserted through rivet holes so that a portion of the rivet shaft projects outwardly beyond the components. Following insertion of the rivet, the preformed head is supported in a support die and the distal end portion of the rivet shaft which projects beyond the components is headed to produce a formed head, sandwiching the components between the formed and premanufactured rivet heads.
Conventionally, the heading of the rivets is performed by either a staking operation or by orbital or radial forming. In staking, a ram driven riveting die is brought into pressure contact with the distal end of the rivet shaft causing the radial deformation of the portion of the shaft which projects beyond the workpiece components. Although the heading of rivets by staking provides a fast and inexpensive method of heading rivets, riveting tools which operate by staking have achieved limited success in controlling the geometry of the formed rivet head.
Increased sophistication of manufacturing processes have more frequently stipulated that formed rivet heads must conform to a specific geometry, as for example, to permit subsequent manufacturing or working of the riveted component parts. To achieve better and more consistent quality of riveted parts, there have been developed processes of heading rivets which involve either the orbital or radial forming of producing a formed rivet head. In orbital forming, a rotating tool holder and shaped tool insert is brought into engagement with the projecting end portion of the rivet shaft. As the rotating tool is brought to bear against the distal end of the rivet, the high speed spinning action of the holder is used to deform the rivet shaft and produce a formed head having a desired shape. Although orbital forming produces a rivet head which may have a uniform predetermined geometry, orbital forming tools are comparatively more expensive and require increased maintenance and riveting production times as compared to conventional staking rivet systems.
The radial formation of a formed rivet head similarly involves a tool holder which is adapted to move through a rosette forming pattern which overlaps the axial centre of the rivet shaft. As a result, the rivet material is spread radially from its axial centre outwardly. As with orbital forming tools, however, radial forming involves the application of complex machinery and increased production times as compared to the heading of rivets by staking.